The comment Damon made about road trip bonding, along with the general look of the car and two handsome brothers on the road to help someone is very much a nod to the popular show, Supernatural.
First appearance of Daniel Gillies as Elijah Mikaelson in The Vampire Diaries universe.
The first mention of Klaus.
Stefan finally apologizes for forcing Damon to turn.
When trying to locate Elena after Bonnie casts the spell, Jeremy states that her location is over 300 miles away. Charlottesville, VA (slightly north of fictional Mystic Falls) to Reidsville, NC where Bonnie's spell tracks Elena, is approximately 150 miles, half of what Jeremy states.